If the bolts/screws that came with the kit doesn’t fit inside the holes in the project board it is probably because the plastic base that came with the Arduino UNO board has not been removed or is used instead of the orange project board.
Start by dismounting the plastic base from the Arduino UNO board. Then insert each bolt through the Arduino UNO board and then through the orange project board from the outer storage box. The screws should now fit. Use three provided nuts to hold the bolts in place. Be careful not to overtighten the nuts.